The concept of using acoustic or seismic waves to determine materials in the soil is old in the art. Typically, an explosion or sudden impact generates an acoustic wave on the surface of the soil and below-ground acoustic detectors measure the received acoustic signals at two or more locations. By measuring the difference in arrival times of the acoustic signal, one determines the average velocities through the soft. If the velocity differs from the expected velocity, the operator is alerted to the fact that the soft lacks homogeneity and can contain objects or contaminates. In contrast, the present invention measures the strength of reflected waves and includes a probe for driving into the soil, with the probe having an acoustic generator and sound detectors to provide an "on-the-go" measurement of materials in the region proximate the probe.
In the present invention, an acoustic generator and multiple acoustic receivers are placed within a cone penetrometer probe to locate objects, soil contaminates and changes in strata where it is likely that pools of soil contaminates may be present.
One object of this invention is to determine the homogeneity or lack of homogeneity of the subsurface soil matrix in a region of approximately 10 meters surrounding the probe. It is known that the measurements performed by various cone-probe sensors is representative of the soil in contact or within a few centimeters of the probe. With such probes the information obtained is meaningful for soils that contact the probe or are extremely close to the probe. However, the information is not necessarily meaningful for all the soil within a few meters of the probe. For example, Dense Non-aqueous Phase Liquids (DNAPL) can permeate the soil in a ganglia structure resulting in filaments of contaminates only a few centimeters thick that extend over a large region. With probes that determine only the soil contaminants contacting the probe or extremely close to the probe, it is possible to miss such soil contaminates unless the probe actually contacts the soil contaminates. For example, Laser Induced Fluorescence produces a measurement of hydrocarbon contamination for a thin strip of soil adjacent to the probe as the probe is being pushed into the soil. In contrast, the present invention detects soil contaminates and changes in strata in regions within approximately 10 meters of the probe. The present invention measures reflected waves to determine changes in time and changes in amplitude of a wave resulting from the presence of materials of different densities in the soil.
Thus the present invention can determine the homogeneity of the surrounding soil in a larger region around the probe as opposed to devices which can only measure soil contaminates in contact with or extremely close to the probe.
In addition to directly locating soil contaminates, the present invention indirectly locates soil contaminates by detecting significant changes in strata, such as fractures and cavities, which are likely to contain underground pools of contaminates. By identifying soil regions likely to contain pools of contaminates, the operator obtains information on ideal placement of the probe to check for the presence of underground pools of contaminates. With the capability to identify underground regions that are likely to contain pools of contaminates, the present invention improves the ability to map a site for contamination more fully and also to potentially decrease the cost of site mapping because the operator knows where to place the probe to look for contaminates.
The operation of the present invention is based upon measurements of the change in reflection of energy of a wave due to the discontinuities in the index of refraction of different underground materials. The discontinuities in the index of refraction result in a change in the velocity of propagation of sound, thereby providing information on the subsurface geology in the region around the probe.